System Administration Guide: IP Services

DHCP Client Event Scripts

You can set up the Solaris DHCP client to run an executable program or script that can perform any action that is appropriate for the client system. The program or script, which is called an event script, is automatically executed after certain DHCP lease events occur. The event script can be used to run other commands, programs, or scripts in response to specific lease events. You must provide your own event script to use this feature.

The following event keywords are used by dhcpagent to signify DHCP lease events:

Event Keyword	Description
`BOUND`	The interface is configured for DHCP. The client receives the acknowledgement message (`ACK`) from the DHCP server, which grants the lease request for an IP address. The event script is invoked immediately after the interface is configured successfully.
`EXTEND`	The client successfully extends a lease. The event script is invoked immediately after the client receives the acknowledgement message from the DHCP server for the renew request.
`EXPIRE`	The lease expires when the lease time is up. The event script is invoked immediately before the leased address is removed from the interface and the interface is marked as down.
`DROP`	The client drops the lease to remove the interface from DHCP control. The event script is invoked immediately before the interface is removed from DHCP control.
`RELEASE`	The client relinquishes the IP address. The event script is invoked immediately before the client releases the address on the interface and sends the `RELEASE` packet to the DHCP server.

With each of these events, dhcpagent invokes the following command:

/etc/dhcp/eventhook interface event

where interface is the interface that is using DHCP and event is one of the event keywords described previously. For example, when the ce0 interface is first configured for DHCP, the dhcpagent invokes the event script as follows:

/etc/dhcp/eventhook ce0 BOUND

To use the event script feature, you must do the following:

Name the executable file /etc/dhcp/eventhook.
Set the owner of the file to be root.
Set permissions to 755 (rwxr-xr-x).
Write the script or program to perform a sequence of actions in response to any of the documented events. Because Sun might add new events, the program must silently ignore any events that are not recognized or do not require action. For example, the program or script might write to a log file when the event is RELEASE, and ignore all other events.
Make the script or program noninteractive. Before the event script is invoked, stdin, stdout, and stderr are connected to /dev/null. To see the output or errors, you must redirect to a file.
Enable the script or program to be run with the following command:
/etc/dhcp/eventhook interface event

The event script inherits its program environment from dhcpagent, and runs with root privileges. The script can use the dhcpinfo utility to obtain more information about the interface, if necessary. See the dhcpinfo(1) man page for more information.

The dhcpagent daemon waits for the event script to exit on all events. If the event script does not exit after 55 seconds, dhcpagent sends a SIGTERM signal to the script process. If the process still does not exit after three additional seconds, the daemon sends a SIGKILL signal to kill the process.

The dhcpagent(1M) man page includes one example of an event script.

Example 16-3 shows how to use a DHCP event script to keep the content of the /etc/resolv.conf file up to date. When the BOUND and EXTEND events occur, the script replaces the names of the domain server and name server. When the EXPIRE, DROP and RELEASE events occur, the script removes the names of the domain server and name server from the file.

Note - The example script assumes that DHCP is the authoritative source for the names of the domain server and the name server. The script also assumes that all interfaces under DHCP control return consistent and current information. These assumptions might not reflect conditions on your system.

Example 16-3 Event Script for Updating the /etc/resolv.conf File

#!/bin/ksh -p

PATH=/bin:/sbin export PATH
umask 0222

# Refresh the domain and name servers on /etc/resolv.conf

insert ()
{
	dnsservers=`dhcpinfo -i $1 DNSserv`
	if [ -n "$dnsservers" ]; then
		# remove the old domain and name servers
		if [ -f /etc/resolv.conf ]; then
			rm -f /tmp/resolv.conf.$$
			sed -e '/^domain/d' -e '/^nameserver/d' \
			    /etc/resolv.conf > /tmp/resolv.conf.$$
		fi

		# add the new domain
		dnsdomain=`dhcpinfo -i $1 DNSdmain`
		if [ -n "$dnsdomain" ]; then
			echo "domain $dnsdomain" >> /tmp/resolv.conf.$$
		fi

		# add new name servers
		for name in $dnsservers; do
			echo nameserver $name >> /tmp/resolv.conf.$$
		done
		mv -f /tmp/resolv.conf.$$ /etc/resolv.conf
	fi
}

# Remove the domain and name servers from /etc/resolv.conf

remove ()
{
	if [ -f /etc/resolv.conf ]; then
		rm -f /tmp/resolv.conf.$$
		sed -e '/^domain/d' -e '/^nameserver/d' \
		    /etc/resolv.conf > /tmp/resolv.conf.$$
		mv -f /tmp/resolv.conf.$$ /etc/resolv.conf
	fi
}

case $2 in
BOUND | EXTEND)
	insert $1
	exit 0
	;;
EXPIRE | DROP | RELEASE)
	remove
	exit 0
	;;
*)
	exit 0
	;;
esac